chore: add mdbook in flake (leptos-rs#1299)

* nix-flake: use follows for `rust-overlay` (..)

This removes the extra nixpkgs dependency by re-using the nixpkgs
already defined.

https://nixos.wiki/wiki/Flakes
https://web.archive.org/web/20230621091703/https://nixos.wiki/wiki/Flakes

> # The `follows` keyword in inputs is used for inheritance.
> # Here, `inputs.nixpkgs` of sops-nix is kept consistent with the `inputs.nixpkgs` of
> # the current flake, to avoid problems caused by different versions of nixpkgs.

* nix-flake: use nix overlays for rustc/cargo (..)

This allows the same nightly rust version to be used across the
flake (vs. strictly in the `devShell`).

* nix-flake: add `mdbook` to the development shell (..)

bumped `nixpkgs` to the latest unstable to pull in mdbook version `0.4.30`.

* nix-flake: expose all rust tools in dev environment (..)

The `oxalica / rust-overlay` docs expose all tools in the development
environment, so we should do the same:

https://github.com/oxalica/rust-overlay#use-in-devshell-for-nix-develop

---------

Co-authored-by: Jay Querie <[email protected]>

flake.nix

@@ -4,6 +4,7 @@
   inputs = {
     nixpkgs.url      = "github:NixOS/nixpkgs/nixos-unstable";
     rust-overlay.url = "github:oxalica/rust-overlay";
+    rust-overlay.inputs.nixpkgs.follows = "nixpkgs";
     flake-utils.url  = "github:numtide/flake-utils";
   };
 
@@ -22,7 +23,8 @@
             openssl
             pkg-config
             cacert
-            (rust-bin.selectLatestNightlyWith( toolchain: toolchain.default.override {
+            mdbook
+            (rust-bin.selectLatestNightlyWith(toolchain: toolchain.default.override {
               extensions= [ "rust-src" "rust-analyzer" ];
               targets = [ "wasm32-unknown-unknown" ];
             }))